1. Technical Field
The present invention relates to data communications with a contactless communication medium, and more specifically, it relates to a data communication system, which is suitable for increasing power supply efficiency depending on modulated waves from a data communication device and for precisely demodulating modulated waves received by a contactless communication medium at the beginning, the data communication device, and the contactless communication medium.
2. Related Art
Conventionally, as a system in which data transmission and reception between an electronic data carrier and-a data communication device, such as a reader or a reader/writer, are performed in a contactless manner, an RFID (radio frequency identification) system is known. In the RFID system, a technique in which the data communication device transmits power for driving the data carrier, together with data, in the contactless manner is known.
In such an RFID system, as a data communication method from the data communication device to the data carrier, two types of Type A and Type B are representatively known. The Type A data communication method is one in which power supply and data transmission are performed through modulated waves which are generated by means of modulation techniques of so-called 100 percent ASK (amplitude shift keying) modulation techniques. The 100 percent ASK modulation techniques associate combinations of modulated waves (modulated waves themselves) on the basis of 0 percent as a modulation degree and modulated waves (no modulated wave) on the basis of 100 percent as the modulation degree with ‘logic 0’ and ‘logic 1’ referred to as pause of digital data to be transmitted.
On the other hand, the Type B data communication method is one in which power supply and data transmission are performed through modulated waves which are generated by means of modulation techniques of so-called 10 percent (actually, in the range of from 8 percent to 14 percent) ASK modulation techniques. The 10 percent ASK modulation techniques associate modulated waves (modulated waves themselves) on the basis of 0 percent as the modulation degree and modulated waves on the basis of 10 percent as the modulation degree with ‘logic 0’ and ‘logic 1’ of digital data to be transmitted. Specifically, the modulated waves on the basis of 0 percent as the modulation degree are associated with ‘logic 0’ and the modulated waves on the basis of 10 percent as the modulation degree are associated with ‘logic 1’.
Moreover, the above-mentioned RFID system is described in detail in RFID Handbook (Japanese Edition), Klaus Finkenzeller, translated by Software Engineering Laboratory Co. Ltd. and published by The Nikkan Kogyo Shimbun, Ltd.
In the Type B data transmission method, however, digital signals generated by a positive logic with respect to transmission data are modulated and transmitted to a data carrier corresponding to the B type data transmission method. Thus, when the data carrier of the Type B performs a demodulation process of modulated waves which are received immediately after the data communication device enters a communication domain, it is apprehended that, in a period until a driving power of a circuit is stabilized after the modulated waves are received, amplitude levels of waveforms of demodulated data signals are not sufficient and thus the waveforms may be corrupted. This is because a digital signal of ‘logic 0’ as a start bit of transmission data is modulated and transmitted in the Type B data communication method.
Accordingly, the present invention has been made in consideration of the unsettled problems in the prior art, and it is an object of the present invention to provide a data communication system which is suitable for increasing power supply efficiency depending on modulated waves from a data communication device and for precisely demodulating modulated waves received by a contactless communication medium at the beginning, the data communication device, and the contactless communication medium.